In a known electric power steering apparatus for a vehicle, rotational output of an electric motor for providing assist steering torque is transmitted, while reduced, to an output shaft of the steering apparatus by a gear apparatus to assist the steering force applied to the steering wheel in the wheel steering operation. In such an electric power steering apparatus, the power is transmitted to the output shaft while the rotation of the electric motor is reduced by a power transmission mechanism provided in a housing.
In the electric power steering apparatus that utilizes the aforementioned power transmission mechanism in the form of a worm gear mechanism, backlash will be present between gear surfaces of a worm and a worm wheel, and consecutive gear slapping sound will be generated at the meshing portion.
Conventionally, due to dimension errors of the worm, worm shaft, bearing portions for supporting it, worm wheel and steering shaft for supporting it that might occur in assembling the worm shaft and worm wheel, backlash occurs after assembly at a relatively high rate. Accordingly, it is necessary to assemble the parts while classifying them in accordance with their degrees of precision. Thus, the assembly process is very complicated.
In view of the above, in a known method, elastic members are provided at both ends of the worm shaft so that the worm shaft is movable in the axial direction, to thereby reduce the impact at the meshing portion and to prevent the gear slapping sound.
According to this method, it is also possible to eliminate play by applying a preload to the bearing for supporting the worm shaft in the thrust direction. In particular, this method can improve delay in assistance due to inertia of the electric motor and enhance information on the road condition to maintain good steering feeling, as is well known.
However, when high output steering assist forces have been achieved, as is the case in recent years, wear of the gear surfaces of the worm and the worm wheel is accelerated, and backlash occurs inevitably. In that case, there is a risk that gear slapping sound due to the backlash may enter the vehicle cabin.
In the above circumstances, there is a known method in which backlash is eliminated by biasing the worm toward the worm wheel (i.e. in the meshing direction) with appropriate means to apply preload.
In applying preload to the worm in the direction toward the worm wheel, it is necessary that tilting (or swinging) of the worm shaft relative to the inner ring or outer ring of the bearing that supports the worm shaft be allowed.
Japanese Patent Application Laid-Open No. 2002-21943 discloses an apparatus in which a projection is formed on a worm shaft inside the inner ring of the bearing that supports the worm shaft so that the worm shaft can be tilted (or can swing) with the projection being the supporting point.
Japanese Patent Application Laid-Open No. 2002-211421 discloses an apparatus in which a self-aligning bearing is used as a bearing for supporting a worm shaft so that the worm shaft can be tilted (or can swing) with the self-aligning bearing being the supporting point.
However, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-21943, since both the worm and the worm shaft are made of a metal, when a radial load is generated in the worm shaft, the worm shaft and the inner ring of the bearing sometimes come in contact with each other in the small gap existing between the worm shaft and the inner ring to generate metallic clank. In addition, it is difficult to control the dimension of the small gap, and it may lead to a significant increase in the manufacturing cost.
Furthermore, since the projection formed on the worm shaft slides on the inner shaft of the bearing, it will be worn and the (small) gap between the worm shaft and the inner ring of the bearing will be enlarged.
In the apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-211421, with the provision of the self-aligning bearing, allowable load in the axial direction of the bearing is low as compared to that of the single row deep groove ball bearing. Accordingly, an increase in the apparatus size is inevitable and a high precision in machining is required, which leads to a significant increase in the manufacturing cost.